Mass spectrometers

Everyone loves a rainbow and most
people understand, at least roughly, how they work:
raindrops split a beam of white sunlight into rays of colored light,
bending the blueish ones more than the reddish ones to make the
well-known arc in the sky. Rain, then, is a brilliant method for
separating sunlight. Chemists and physicists use a similar method for
separating mixtures of substances into their components, turning them
into beams of particles and then bending them with electricity and
magnetism to make a kind of spectrum of different atoms that are
easier to identify. This technique is called mass spectrometry
and it was pioneered by British physicist Francis Aston in 1919.
Let's take a closer look at how it works!

Photo: Rainbows bend short wavelength blue light more than long-wavelength red light. Mass spectrometers work in a very similar way.

What is a mass spectrometer?

Mass spectrometers are much simpler than they look—or sound. Suppose someone gives you
a bucketful of atoms of different chemical elements and asks you
what's inside. You need to separate out the atoms quickly and
efficiently, but how do you do it? Simple! Tip your bucket into a
mass spectrometer. It turns the atoms into ions (electrically charged
atoms with either too few or too many electrons). Then it separates
the ions by passing them first through an electric field, then
through a magnetic field, so they fan out into a spectrum. A computerized
detector tallies the ions in different parts of the spectrum and you can use this information to figure out what
kinds of atoms were originally in your bucket. That's the basic idea,
anyway. In reality, it's a bit more complex than this—there's no bucket, for a start!

Photo: A scientist uses a mass spectrometer in the Aeronomy Laboratory, Air Force Geophysics Laboratory (AFGL).
Photo by William W. Magel courtesy of US Air Force and Defense Imagery.

How does a mass spectrometer work?

There are numerous different kinds of mass spectrometers, all working in
slightly different ways, but the basic process involves broadly the
same stages.

You place the substance you want to study in a vacuum chamber inside the machine.

The substance is bombarded with a beam of electrons so the atoms or molecules it
contains are turned into ions. This process is called ionization.

The ions shoot out from the vacuum chamber into a powerful electric field (the
region that develops between two metal plates charged to high
voltages), which makes them accelerate. Ions of different atoms have
different amounts of electric charge, and the more highly charged
ones are accelerated most, so the ions separate out
according to the amount of charge they have. (This stage is a bit
like the way electrons are accelerated inside an old-style,
cathode-ray television.)

The ion beam shoots into a magnetic field (the invisible, magnetically active
region between the poles of a magnet). When moving particles with an
electric charge enter a magnetic field, they bend into an
arc, with lighter particles (and more positively charged ones) bending more than heavier ones
(and more negatively charged ones). The ions
split into a spectrum, with each different type of ion bent a
different amount according to its mass and its electrical charge.

A computerized, electrical detector records a spectrum pattern showing
how many ions arrive for each mass/charge. This can be used to
identify the atoms or molecules in the original sample. In early
spectrometers, photographic detectors were used instead, producing a
chart of peaked lines called a mass spectrograph. In
modern spectrometers, you slowly vary the magnetic field so each separate
ion beam hits the detector in turn.

How does it work in reality?

Although that's a very simplified explanation, it's not too far from what really happens.
Take a look at this drawing of an early mass spectrometer designed by American
electrical and electronic engineer
Dr Robert V. Langmuir [PDF]. in the late 1930s and patented in 1945. I've colored
it to make it easier to follow and used the same numbering as I used up above to
emphasize the similarity. Here's how it works:

A sample of gas (blue) flows into the vacuum chamber (inner orange circle).

The sample is bombarded with electrons to make ions.

The ions are accelerated downward in an electric field (toward the curved electric plate labelled 3).

A magnetic field created by the electromagnet (outer red circle) bends the ions round in a semicircle (yellow).

The ions separate out and are picked up by the electronic detector apparatus (green).

Artwork: Mass spectrometer designed by Robert Langmuir. Diagram courtesy of US Patent and Trademark Office.

What is mass spectrometry used for?

Like chromatography, with which it's often paired, mass spectrometry is an
important method for identifying the atoms or molecules in complex
chemical substances. The inventor of the spectrometer,
Francis Aston (1887–1945), used his machine to prove the existence of
many naturally occurring isotopes (atoms of
the same element with different numbers of neutrons and different
mass). Mass spectrometry is also widely used in
forensic science
(to identify samples found at crime scenes), by materials scientists
(for example, to study impurities in steel), and with radio-carbon
dating to calculate the approximate age of important deposits
unearthed by archaeologists.

Can technology stop terror in the air? by Steven Ashley, Popular Science, November 1985. This old but excellent article from Pop Sci explores the various technologies that were being developed to stop terrorist attacks on airplanes back in the 1980s, including X ray machines and mass spectrometers.

Patents

If you're looking for a detailed technical description of how mass spectrometers work, patents are a really good place to start. Here are a few I've picked out from Google Patents:

US Patent #2,370, 673: Mass Spectrometry by Robert Langmuir, Consolidated Engineering Corporation, March 6, 1945. The earliest mass spectrometer patent I've found. Dr Langmuir went on to work for General Electric's Research Laboratories before joining Caltech's Electrical Engineering department.